Halogen-containing polymers, such as polyvinylchloride (PVC), are some of the most widely used plastics in the world. PVC in particular is widely used in such applications as pipes and pipe fittings, film and sheet, flooring, cables and construction profiles. However, PVC can decompose during processing, upon heating or on prolonged exposure to sunlight due to loss of HCl from the polymer, resulting in discoloration and embrittlement. Alkyltin stabilizers are known to be particularly effective in preventing a discoloration of halogen-containing polymers at elevated temperatures, for example, “PVC Degradation and Stabilization,” Wypich, George; ChemTec, Toronto 2008. “Handbook of Vinyl Formulating”, 2nd edition; Grossman, Richard F.; Wiley & Sons, 2008. “PVC Handbook”; Wilkes, Charles E., et al; Hanser, Cincinnati 2005. However, alkyltin stabilizers are also known to have varying toxicity levels, as well as differing volatility and efficiency characteristics. These features are important considerations when choosing a stabilizer since the overall toxicity is related not just to the specific amount of the stabilizer and tin used in PVC compounds but also to its volatility. This directly correlates with a level of personnel exposure during handling of the alkyltin stabilizer and processing of PVC stabilized with those materials. There is a need for halogen-containing compositions containing alkyltin heat products that are not only efficient stabilizers, but have a reduced toxicological impact.
Several approaches have been used in an effort to develop non-toxic organotin heat stabilizers suitable for halogen-containing polymers. These approaches include: A) preparation of mono-alkyltin stabilizers while minimizing the content of both di-alkyl and tri-alkyltin compounds; B) preparation of mono-/di-alkyltin mercaptoacetate ester blends, where the alkyl group contains 12 carbon atoms, and the mercaptoacetate ester ligand is obtained with n-alcohols containing 8 carbon atoms or less; and C) preparation of mono-/di-alkyltin mercaptoacetate ester blends, where the alkyl group contains 12 carbon atoms, and the mercaptoacetate ester ligand is obtained with n-alcohols containing more than 8 carbon atoms.
EP 2123659 discloses high purity mono-alkyltin compounds containing mono-alkyltin compounds of 95-99.99% purity, having 0.01-0.5% dialkyltin compounds and 0.01-0.5% tri-alkyltin compounds, and a process for making the high purity mono-alkyltin compounds. The heat stabilizers are used in chlorine-containing polymers, glass coating chemicals, catalysts and articles comprising of at least one polymer and the high purity mono-alkyltin compound.
U.S. Pat. No. 8,198,352 is a modified version of EP 2123659, where the purity of the mono-alkyltin compounds ranges from 85 to 99.999%, and the di- and tri-alkyltin compounds are present at 0.001-10% and 0.001-5%, respectively.
U.S. Pat. No. 4,496,490 discloses preparation of high purity mono-octyltin mercaptoacetate heat stabilizers starting from a mono-octyltin chloride of 99.2% purity. While the product contained up to 5% tri-octyltin iso-octylmercaptoacetate, the presence of di-octyltin compounds in the final product was not reported.
U.S. Pat. No. 4,193,913 discloses high purity mono-alkyltin stabilizers that were prepared using mono-methyl-, mono-butyl- or mono-octyltin chlorides as raw materials, and reacting those with mercaptoacetate esters. The purity of the chlorides was not specified.
EP 1743898 discloses preparation of mono- and dialkyltin chlorides. While di-octyltin chloride was obtained at a purity of greater than 98%, purity of the mono-alkyltinchloride was not measured.
EP 1225177 discloses preparation of mono-alkyltin halides at greater than 60% yield using a variety of catalysts. However, purity of the obtained products was not measured.
GB 1510973 discloses the preparation of mono-octyltin mercaptoacetate stabilizers using mono-octyltin chloride of 99.2% purity. The purity of the prepared mono-octyltin stabilizer was not measured or disclosed.
EP 2123659, U.S. Pat. Nos. 8,198,352, 4,496,490 and 4,193,913 also disclose that mono-octyltin stabilizers are of low toxicity. However, it should be noted that high purity mono-alkyltin stabilizes are of lower tin content (see Table 1), and therefore, inefficient in terms of providing adequate long-term heat stability to PVC compared with the synergistic blends containing both mono- and di-alkyltin mercaptoacetates.
TABLE 1Calculated Tin content of selected mono- and di-2-ethylhexylmercaptoacetatesMolecularMono-/di alkyltin 2-ethylhexyl mercaptoacetatesWeight% TinMono-methyltin tris(2-ethylhexyl mercaptoacetate)74416.0Di-methyltin bis(2-ethylhexyl mercaptoacetate)55521.4Mono-butyltin tris(2-ethylhexyl mercaptoacetate)78615.1Di-butyltin bis(2-ethylhexyl mercaptoacetate)64018.6Mono-octyltin tris(2-ethylhexyl mercaptoacetate)84214.1Di-octyltin bis(2-ethylhexyl mercaptoacetate)75215.8
U.S. Pat. No. 4,988,750 discloses preparation of a heat stabilizer comprising a 60/40 mono- and di-dodecyltin mercaptoacetate ester blend, where alcohols used for obtaining the mercaptoacetate ligand contained 8 carbon atoms.
U.S. Pat. No. 3,769,263 discloses dibutyltin mercaptoacetate and mercaptopropionate esters which were used as co-stabilizers, where the mercapto-ester ligands were obtained from alcohols containing 12 and 18 carbon atoms.
Nevertheless, a continuing need exists for organotin heat stabilizers for halogen-containing polymers having good efficiency and reduced toxicological impact during handling of the stabilizers and processing of PVC stabilized with those materials. It has unexpectedly been found that heat stabilizer compositions comprising particular blends of di-, mono- and tri-methyltin compounds and a thioester provide excellent heat stability for halogen-containing polymers, while also possessing reduced toxicological impact arising from lower volatility of the stabilizers.